1. Field of Invention
This invention relates to an image processing method for reducing the amount of marking material required to print a colored image, to avoid problems common to the use of excessive amounts of ink. In particular, this invention relates to an image processing method in which the multiple ink drop sizes are restricted and adjusted so as to maintain a total ink level within desired limits, while retaining the printed image color substantially unchanged and providing a high quality printed image.
2. Description of Related Art
Full color printing has become a desired goal of office, as well as home, products. One type of full color printer that has significant potential for fulfilling such a goal is the ink jet printer, due primarily to its low purchase and operating costs combined with its high quality output. In one common design of such printers, a reservoir of liquid ink is connected to an ink output orifice via a capillary tube. In the tube, a heater element is provided, responsive to an on/off or binary printing signal. When printing is required and a printing signal is directed to the heater element, the heater element rapidly heats ink in the capillary tube adjacent thereto to a gaseous state, producing a pressure differential that expels a droplet of ink from the orifice, directing the droplet to a sheet of receiving material, such as paper. Color printing is accomplished by providing multiple layers or separations of ink on the page. Commonly, colors are provided by subtractive combinations of cyan, magenta and yellow inks. To print black, a combination of equal amounts of cyan, magenta and yellow is printed, or a fourth black ink is used as a substitute. Undercolor removal, a well known process in the printing arts, can be used to print a single layer of black ink as a substitute for the combination of equal amounts of cyan, magenta and yellow. For a fuller discussion of under color removal and its application to electronically derived or created images, reference is made to J.A.C. Yule, Principles of Color Reproduction, (John Wiley & Sons, Inc., New York, 1967), pages 294-327. Other full color printing processes may use dry powder or liquid toners.
A problem of ink jet printers is that the liquid inks used have a finite drying time, which tends to be somewhat longer than desirable. Further, the drying time of any particular area is at least partly a function of the amount of ink deposited on that area. That is, as the amount of ink deposited in a given area increases, the drying time of the ink in that area also generally increases. While satisfactory drying times are possible with black-only or single separation printing, once multiple separations are required, the large amount of liquid on the page causes the problems of ink puddling or pooling, bleeding to adjacent image areas, and flow through to the back side of the receiving material. Paper cockle is also a problem due to saturation of the paper receiving material and subsequent rapid drying. Particularly, problems are noted in the printed image at high ink coverage areas, and high coverage areas where high contrast image edges occur. While certain materials variations, such as selection of different inks or the use of special papers, may resolve some of these problems, each brings its own distinct problems to the process. While special treated papers optimized for ink jet use are possible, plain papers are preferred from cost and convenience standpoints.
Using one available set of materials, a maximum ink coverage of about 150% is required for printing without artifacts resulting from too much ink. As used herein, ink coverage refers to the number of ON pixels in a region for all the separations, divided by the total number of pixels in the region in one separation. Without undercolor removal, a typical full color image may require ink coverage in the range of 200-300%. With undercolor removal, maximum ink coverage may be down to 200%, but no lower. Additionally, it would be highly desirable for the process to operate irrespective of image content, or on the separation binary bitmaps without further image information.
While ink jet printing has a notable problem with the case of high ink coverage, other printing processes that are printed on a pixel by pixel basis also have problems with excessive marking material. Notably, electrophotographic printing methods using a pixel by pixel printing process for forming a latent image for development by dry or liquid toner marking materials, can suffer from excessive marking material, evidenced by sheet cockling, and curling caused by differential shrinkage of toner and paper in the printing process.
U.S. Pat. No. 4,930,018 to Chan et al. teaches the reduction of paper cockle and graininess of ink jet prints. Printing of a given scan line occurs multiple times, with three different dye loadings, with pixels requiring the highest dye loading printed on one pass, pixels requiring an intermediate dye loading printed on another pass, and pixels requiring the lowest dye loading on another pass. The method takes as input continuous tone RGB (red-green-blue) images and performs RGB-CMYK (cyan-magenta-yellow key or black) conversion with full under color removal. As understood, printing is performed at half resolution, so that "pixels" in the input image correspond to 2.times.2 blocks in the output image. The image data is first error diffused from 8 bits per pixel per separation to 4 bits per pixel per separation. Then, for each pixel, a count of up to 4 drops of each dye loading is computed, for each separation. There are multiple choices, ranked in order of total ink coverage. If the highest coverage choice exceeds the maximum allowable coverage, the separation with highest coverage is changed to use a lower coverage value for the same gray level, if possible. If it is not possible to stay at the same gray level, the gray level for that separation is dropped by one, and the error passed on to neighbors. The process iterates until the total ink coverage is as low as required. Pixels within the 2.times.2 block are assigned values (0 or 1) by proceeding around the block in clockwise order, and filling in pixels in order. First, the high dye load pixels are turned on, then the medium, then the low. Within each dye loading group, first black is turned on, until there are no more black pixels of that dye loading, then the next pixels in the cycle are turned on, until there are no more cyan required, then magenta, and yellow, and then the next dye load group. By maximizing ink coverage and using multiple dye loadings, they reduce the noisiness of the image, and by maintaining the total ink coverage within known limits, they prevent the many problems associated with excessive ink.
U.S. Pat. No. 4,999,646 to Trask teaches limiting coverage to 100% coverage (by the above definition of coverage), or perhaps between 100 and 200% coverage (if 100% corresponds exactly to no white spaces on a page), owing to the circular shape and overlap of print dots. Coverage is limited by using 2.times.2 super pixels and assigning each one drop per pixel in a combination that depends on the color required. Assuming one bit per separation input with full undercolor removal, there are eight possible colors that could be requested (including white). In order to reduce patterning due to the multiple swaths, two passes are used, each of a checkerboard pattern of pixels (the two passes being offset to provide full coverage). The two pass process allows ink to dry between passes.
U.S. Pat. No. 5,237,344 to Tasaki et al. describe a method for reducing the amount of ink printed to 50%, 75% or 66%. The method uses fixed patterns of turn-off locations (e.g., a checkerboard for 50%) and selects the pattern based on the printing mode (reverse character mode, block graphic mode or normal character mode), the character selected, and possibly the relative humidity. Apparently, the method is designed for single color (black) printing: if it were used for multiple separation (e.g., red formed from yellow and magenta) printing, both separations would be turned off in the same place, resulting in more obvious patterns. The small set of fixed turn-off patterns makes the method very sensitive to line angle, as lines at some angles will have more pixels turned off than others. Also the method is only useful for characters from a built-in font, including graphic characters; arbitrary fonts and shapes, such as are requested in documents created using industry standard page description languages, such as PCL or PostScript, cannot be handled in this way.
Other methods for reducing marking material coverage have been developed, and have become generally known as color reduction techniques. In these techniques, an algorithm is generally used to turn OFF pixels in an image when printing at the pixel would result in excessive ink coverage. Thus while ink coverage is reduced on a pixel-by-pixel basis, the overall appearance of the image is not substantially degraded.
U.S. Pat. No. 5,515,479 to Klassen, the entire disclosure of which is incorporated herein by reference, discloses a color reduction method for processing images for printing. The method includes the steps of, for each separation, making a pixel level determination of marking material coverage, based on the number of pixels within a given area that are turned on. If a determination is made that too much marking material will be placed within the given area, a fraction of pixels in the area are turned off to reduce the amount of marking material that will be used for the given area. The method uses a processing order through each given area to prevent artifacts from occurring in the pixel reduction step, which tends to randomize the turn off effect. Additionally, in the disclosed method, the results of the determination are compared for each separation, to determine that the turnoff result allows at least one corresponding pixel among each of the separations to be turned on.
Other color reduction methods are also described in U.S. Pat. Nos. 5,649,071, 5,635,967, 5,563,985, and 5,519,815, the entire disclosures of which are incorporated herein by reference. U.S. Pat. No. 5,649,071 to Klassen et al. discloses a method of processing images preparatory to printing in a color printer adapted to printing serial color separations, that when superposed form the final color image. In the disclosed method, separation color images are received for printing defined by continuous tone separation signals, which are color corrected and halftoned preparatory to printing the color image. The received continuous tone separation image signals are used to derive an estimate of marking material coverage. The marking material coverage estimate is subsequently used in association with random number generation to turn OFF otherwise ON pixels in each separation as the separation is printed for the reduction of marking material coverage.
U.S. Pat. No. 5,635,967 to Klassen discloses a method and apparatus for reducing marking material coverage in reproduction of edges in a halftoned image. The method includes the steps of: a) detecting edges in the image using edge filtering; and b) reducing marking material coverage at the detected edges. Interseparation correlation may also be provided, for the purpose of ascertaining whether a detected edge is against a white or no colorant region, in which circumstance, marking material coverage is not reduced. The patent further describes the reduction of marking material coverage at the detected edges in proportion to the overall coverage in the image region, taking into account the kind of colors (primary or secondary) forming the edge.
U.S. Pat. No. 5,563,985 to Klassen et al. discloses a method for processing images for printing color reduced images. The method includes the steps, for each separation, of making a pixel level determination of marking material coverage, based on the number of pixels within a given area that are turned on. If a determination is made that too much marking material will be placed within the given area, a random number function is used to turn off a fraction of pixels in the area to reduce the amount of marking material that will be used for the given area. To prevent artifacts from occurring in the pixel reduction step, a processing order through each given area is used that tends to randomize the turn off effect. Additionally, the results of the determination are compared for each separation, to determine that the turnoff result allows at least one corresponding pixel among each of the separations to be turned ON.
Finally, U.S. Pat. No. 5,519,815 to Klassen discloses a method and device for processing color images preparatory to printing to effect color reduction. The method of reducing marking material coverage in text and line art areas of secondary colors includes the steps of: a) determining the locations of text and line art color pixels having excessive marking material coverage; b) upon determining the locations of color pixels having excessive marking material coverage, processing separation pixels to turn OFF a predetermined portion of the separation pixels corresponding to color pixels having excessive marking material coverage; and c) to prevent artifacts from occurring in the pixel reduction step, processing a given area of separation pixels in an order which tends to randomize the turn OFF effect.
Ink jet printers capable of printing multiple drop sizes are generally known in the art. For example, U.S. Pat. No. 5,412,410 to Rezanka discloses a thermal ink jet printhead, which has two or more groups of selectively activatable heating elements and associated nozzles within each group having the same geometric parameters, but the geometric parameters and nozzles between groups being different so that the ejection from the nozzles of different groups have different ink volumes. Although this patent discloses that the printhead can be used to provide higher quality images, such as grey scale printing and high resolution text printing, the patent does not teach or suggest using the different drop volume nozzles as a means for reducing marking material coverage.